Various methods for manufacturing a hollow engine valve have been developed, and there have been methods which mold a hollow engine valve by forging. For example, Patent Document 1 discloses a method for manufacturing a valve head part of a hollow engine valve and a hollow engine valve.
The manufacturing method described in Patent Document 1 obtains a hollow engine valve body by: molding a cup-shaped intermediate member by boring a columnar hole in the upper surface of a solid circular rod (a columnar billet) by hot forging with a punch; molding a valve head part forming part by widening a lower portion of the rod by forging; and performing necking on the rod multiple times to gradually narrow an upper portion of the valve head part forming part and thereby form a valve head part and a hollow stem part connected thereto.